A LED lighting system includes a switching regulator and a LED, wherein the switching regulator has a controller therein. The switching regulator is controlled by the controller thereof to provide a current to the LED. The controller includes components to calculate an actual charge amount delivered to the LED system according to the current and an active time period of an LED current time period, wherein the LED current time period is duty cycle modulated at a rate of greater than fifty (50) Hz and to utilize the actual charge amount to modify and provide a desired target charge amount to be delivered during a future active time period of the LED current time period.
The LED system further has components to calculate the active time period of the LED current time period an actual charge amount delivered to the LED system, and also has components to compare the actual charge amount with a desired charge amount for the active time period of the LED current time period, and to compensate a difference between the actual charge amount and the desired charge amount during the future active time period. In order to accurately control the desired charge amount, the average LED current should be controlled well, and the light intensity of the LED should be effectively controlled.
Unfortunately, some conventional switching regulator used in the LED system has a logic gate and a gate driver (or called pre-driver), and thus a delay issue occurs, such that the average LED current cannot be controlled easily. Furthermore, the inductance of the inductor used in the switching regulator and the ratio of the input voltage and the output voltage (i.e. regulator output) of the LED lighting system may also affect the control of the average LED current.
Please refer FIG. 1. FIG. 1 is a block diagram of a conventional LED lighting system. The conventional LED light system comprises a switching regulator and a LED 12, wherein the switching regulator is a buck type regulator, and comprises a controller 10, a first current sensing resistor 11, a second current sensing resistor 13, a Zener diode 14, and an inductor L. The controller 10 is connected to the first current sensing resistor 11, the LED 12, the second current sensing resistor 13, the Zener diode 14, and the inductor L. One end of the first current sensing resistor 11 is used to receive an input voltage Vin, and other one end of the first current sensing resistor 11 is connected to one end of the inductor L. Anode of the Zener diode 14 is connected to other one end of the inductor L, and cathode of the Zener diode 14 is connected to one end of the second current sensing resistor 13. Anode and cathode of the LED 12 are respectively connected to other one end of the second current sensing resistor 13 and a ground GND.
The controller comprises an error signal generation circuit 101, a fast current monitor 102, a hysteresis controller 103, a resistor 104, a hysteresis comparator 105, a gate driver 106, and a switching NMOS transistor 107. The fast current monitor 102 is parallel connected to the first current sensing resistor 11 to sense a voltage across the first current sensing resistor 11 and generate a first sensing current accordingly. The error signal generation circuit 101 is parallel connected to the second current sensing resistor 13 to sense a voltage across the second current sensing resistor 13 and generate an error signal accordingly, wherein the error signal represents the error between a desired regulator output and an actual regulator output.
The hysteresis controller 103 is connected to a negative input terminal and an output end of the hysteresis comparator 105, and an positive input end of hysteresis comparator 105 and the hysteresis controller 103 receive a reference voltage Vref, such that the hysteresis controller 103 and the hysteresis comparator 105 form a hysteretic comparator for controlling the on/off state of the switching NMOS transistor 107. The negative input end of the hysteresis comparator 105 is further to receive a voltage across the resistor 104, wherein two ends of the resistor 104 are respectively connected the negative input end of the hysteresis comparator 105 and the ground GND, and the voltage across the resistor 104 is formed according to the error signal, the first sensing current, and the hysteresis control signal output by the hysteresis controller 103. The gate driver 106 is used to receive the output signal of the hysteresis comparator 105 to generate a driving signal to a gate end of the switching NMOS transistor 107. A drain end and a source end of the switching NMOS transistor 107 are respectively connected to the other one end of the inductor L and the ground GND.
The error signal generation circuit 101 comprises an integrator 1011, an accurate current monitor 1012, a subtracting unit 1013, and a demand current source 1014. The accurate current monitor 1012 is parallel connected to the second current sensing resistor 13 for sense the voltage across the second current sensing resistor 13, so as to generate a second sensing current accordingly. The subtracting unit 1013 are connected to the integrator 1011 and the demand current source 1014, and used to subtract the demand current from the second sensing current to generate the subtraction current to the integrator 1011. The demand current source 1014 is used to receive the reference voltage Vref to generate a demand current accordingly. The integrator 1011 is further connected to the negative input end of the hysteresis comparator 105, and used to integrate the subtraction current to generate the error signal. The error signal is the integration of the subtraction of demand current and the second sensing current, and this means the error signal represents the error between the desired regulator output and the actual regulator output.
It is noted that though the conventional the switching regulator and its controller 10 can solve the above problems for affecting the average LED current, two current monitors (i.e. the fast current monitor 102 and the accurate current monitor 1012) are required, and thus increasing the circuit hardware cost, especially the accurate current monitor 1012. A simpler circuit of the switching regulator and its controller are still demanded by the market.